I did a chemistry experiment about Aspirin synthesis and recrystalization to get

Question

I did a chemistry experiment about Aspirin synthesis and recrystalization to get pure aspirin

This is my experimental result of melting point of Recrystallized aspirin and crude aspirin.

I have some questions.

1. How can I explain if my sample weighted more theoretical yield of aspirin? Can you tell me why percent yeield of my crude aspirin has defferent results from the results obtained from the re-crystallized product?

2. I want to know about the purpose of the Ferric chloried test and what you exepect to be the impuriteis in my sample. This is the result of Ferric chloride test. Please explain it in detail.

3. Please explain any difference between the melting points of the crude aspirin and re-crystallized samples of aspirin. Please make estimation of the purity by ensuring the melting point range.

This is my results.

Substance

Melting point

Commercial aspirin

138-140?

Salicylic acid

158-160?

4. Finally I want to know about any sources of error in this experiment.

Substance

Melting point

Commercial aspirin

138-140?

Salicylic acid

158-160?

Explanation / Answer

1. How can I explain if my sample weighted more theoretical yield of aspirin? Can you tell me why percent yeield of my crude aspirin has defferent results from the results obtained from the re-crystallized product?

This is due to the presence of impurities. The crude product may contain salicylic acid , as well as water or acetic acid as impurities. Salicylic acid can be formed either from incomplete reaction or from hydrolysis of the product during its  isolation. Additional water molecules can come from the improper drying of the crude product. It is quite common that your crude has impurities. The recrystallization process nothing but purification process. After this process the yield of the reaction will decrease as you are removing impurities from the crude product.

2. I want to know about the purpose of the Ferric chloried test and what you exepect to be the impuriteis in my sample. This is the result of Ferric chloride test. Please explain it in detail.

The main impurity in Aspirin synthesis is salicylic acid, and it can come from  either from incomplete reaction or from hydrolysis of the product during its  isolation. The purpose of Ferric chloried test is to confirm/rule out the presence of salicylic acid in the Aspirin.

Only the salicylate ion complexes to iron(III) (violet colour) due to the presence of phenolic acid group. The recrystallized aspirin, an ester, should not give any positive result to the tests since esters do not react with FeCl3. The violet colour intensity represents the amount of salicyclic acid present in the sample. The fact that your recrystallized compound is still pale violet indicates presence of small amount of salicyclic acid. Pure (100%) should give yellow colour similar t blank. Second time recrystallization will help to remove this impurity also.

3. Please explain any difference between the melting points of the crude aspirin and re-crystallized samples of aspirin. Please make estimation of the purity by ensuring the melting point range.

In general, pure products will sharp melting point (with a range of 2 oC) . If impurities are present in your sample, the melting point range for your product will be lower than the range of pure aspirin. Your observed values for both crude and recrystallized are lower than expected values and have wider range (>10 oC) indicates presence of significant amount of impurities in your compound.

4. Finally I want to know about any sources of error in this experiment.

It is clear from your results that you have significant amount of salicyclic acid in your product. It can come from either from incomplete reaction or from hydrolysis of the product during its  isolation as pointed out earlier.

Incomplete reaction can be due to several reasons such as improper mixing of reagents, error in the reaction temperature etc.

Hydrolysis of the product during its isolation: This can happen if you leave the product very long time in water during isolation.

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